getCredentials

Extracts the credentials from the given servlet request.
Build a Credentials object for the given authorization header.
The creds may be used to login to the repository. If the specified header
string is null or not of the required format the behaviour
depends on the defaultHeaderValue field:

if this field is null, a LoginException is thrown.
This is suiteable for clients (eg. webdav clients) for with
sending a proper authorization header is not possible, if the
server never send a 401.

if this an empty string, null-credentials are returned, thus
forcing an null login on the repository

if this field has a 'user:password' value, the respective
simple credentials are generated.